update virtual environment instructions

[cs01]

Now that the venv module is part of the standard library for all non-EOL'd Python versions, the instructions can be simplified to use venv rather than virtualenv.

This PR replaces mentions of virtualenv with venv everywhere but the section that refers users to virtualenv if they are using an older Python version. virtualenv-related tools were removed.

[jhermann]

• Maybe mention the need to still use python3 on many Linux releases.
• Similarly py -3 -m venv … on Windows.
• Mention using tox to manage venvs in a project workdir?

[cs01]

• Mentioned python might point to python2
• Mention py instructions for windows
• Mention tox, nox, pipx

[jhermann]

--no-site-packages is the default and deprecated for a long time now – actually so long it now disappeared from the newest releases. The --system-site-packages option has to be described instead.

[cs01]

This is ready for another review